ASN lookup

Enter an IP address and see the autonomous system that announces it, its owner and the IP range.

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What the ASN lookup does

Enter an IP address and the lookup returns the autonomous system that announces it: the AS number, the organization name, the network prefix, the country and the regional registry that allocated it. The data comes from Team Cymru's IP-to-ASN service, which maps each IP to the origin AS seen in global BGP routing.

How to read it

An autonomous system is a network run under one routing policy, like a cloud provider, an ISP or a large company. The prefix is the block the IP belongs to, as announced on the internet. The registry is one of the five regional internet registries: ARIN, RIPE NCC, APNIC, LACNIC or AFRINIC.

The AS name tells you who runs the network, which isn't always who uses the IP. A cloud provider's AS covers thousands of customers.

When you need it

Identifying a source IP in a DMARC report that has no clear reverse DNS.
Checking whether a sending IP sits in a home, hosting or business network.
Understanding a blocklist that lists whole networks.
Tracing abuse or odd traffic in your logs.

Why it matters for cold email

When DMARC reports show an unknown source sending as your domain, the ASN is often the fastest answer. A Google or Microsoft AS usually means one of your own inboxes or a tool built on them. An unexpected hosting network may be a forgotten tool, or someone spoofing you.

It matters for your own servers too. Some blocklists, such as UCEPROTECT level 3, list entire ASNs, so your host's network reputation can affect your IPs.

Questions

What is an ASN?

An Autonomous System Number identifies a network that controls its own routing on the internet, such as an ISP, a cloud provider or a large company. Each AS announces the IP prefixes it's responsible for through BGP.

How do I find the ASN of an IP address?

Enter the IP here. The lookup returns the origin AS number, its name, the announced prefix, the country and the registry.

What are Google's and Microsoft's ASNs?

Google's main network is AS15169 and Microsoft's is AS8075. Both run other ASNs for specific services, so look up the IP itself rather than assuming.

Is an ASN the same as an ISP?

Not quite. Every ISP has at least one ASN, but so do cloud providers, universities, content networks and large companies, and some organizations run many.

Why does an IP show a different country than I expected?

The country comes from the registry allocation, which records where the block was assigned, not where the server sits. A provider can use addresses registered in one country for servers in another.